Future LTE-A (Long Term Evolution-Advanced) system will support a transmission bandwidth up to 100 MHz. However, the maximum supportable transmission bandwidth in LET (Long Term Evolution) standard is 20 MHz, and thus it is necessary to aggregate multiple carriers to achieve wider transmission bandwidth. Carrier aggregation is a technique proposed by 3GPP (3rd Generation Partnership Project) to aggregate multiple carriers for joint transmission, so as to support the higher requirement on the transmission bandwidth by the future mobile communication systems. According to the position of the carriers to be aggregating in the frequency spectrum, the carrier aggregation can be classified into continuous carrier aggregation and non-continuous carrier aggregation, and LTE-A simultaneously supports both of the two aggregation scenes. While introducing the carrier aggregation technique, 3GPP also takes into consideration of the backward compatibility. This means that for a long period of time in the future, terminals that support carrier aggregation and terminals that do not support carrier aggregation will coexist, the terminals that support carrier aggregation can access multiple carriers at the same time, and the terminals that do not support carrier aggregation can access only one carrier.
To simplify the radio resource management in the carrier aggregation scene, a concept of primary component carrier (PCC) will be introduced into LTE-A. Therefore, the carrier management in the carrier aggregation scene will develop from the distributed management towards the centralized management. Thus, the primary component carrier will necessarily have functions a common carrier does not have, playing an important part in the radio resource management.
Naturally, the carrier corresponding to the cell for the initial access of the terminal is selected as the primary component carrier. However, with the increasing requirement on the quality of service by the user, it may be necessary to add a new carrier to form the carrier aggregation. With the variation in the terminal, the base station and the network conditions, it is also possible to replace or delete some carriers in use. Because of the movement of the user and the difference between the performances of the carriers that are aggregating, it may be necessary to re-designate the primary component carrier.